Various techniques for speakers have been studied. For example, a technique using an oscillation device that oscillates ultrasonic waves may be mentioned. A highly directional sound field can be formed by using an oscillation device, which oscillates ultrasonic waves, for sound reproduction of a speaker. As a technique for a speaker using ultrasonic waves, for example, techniques disclosed in Patent Documents 1 to 4 may be mentioned. The technique disclosed in Patent Document 1 is to form an audible region, which can be moved arbitrarily, by controlling the frequency or phase of the oscillating ultrasonic wave.
The technique disclosed in Patent Document 2 controls the directivity by providing a plurality of speakers, which oscillate ultrasonic waves, inside the speaker cabinet and changing a sound signal to drive each speaker. The technique disclosed in Patent Document 3 is related to a super directional speaker including a unit that detects a target person for whom the sound is to be output. Patent Document 3 discloses that high advertising effect is obtained accordingly since the sound can be output with a person having a certain attribute as a target. The technique disclosed in Patent Document 4 is related to a directional speaker including a display unit that displays the output range of sound.